Sheet beam electron discharge device with means for preventing unwanted oscillations of the beam



Jan. 28, 1969 w. L. CARL 3,424,933

. SHEET BEAM ELECTRON DISCHARGE DEVICE WITH MEANS FOR PREVENTING UNWANTED OSCILLATIONS OF THE BEAM Filed Aug. 8, 1966 v INVENTOR. ILLIAM L. CARL as ATTORNEY United States Patent 3,424,933 SHEET BEAM ELECTRON DISCHARGE DEVICE WITH MEANS FOR PREVENTING UNWANTED OSCILLATIONS OF THE BEAM William L. Carl, Owensboro, Ky., assignor to General Electric Company, a corporation of New York Filed Aug. 8, 1966, Ser. No. 570,818 US. Cl. 31383 3 Claims Int. Cl. H01j 29/46 ABSTRACT OF THE DISCLOSURE An electron beam discharge device is provided comprising an electron gun including an electron emitting cathode; an anode for receiving electrons emitted from said cathode; a plurality of focusing electrodes having apertures arranged in a series between said cathode and anode to shape the electrons into an electron beam; accelerator means in the path of said beam including a limiter grid in said beam and delineating in said beam a first electron beam area adjacent the cathode and a second electron beam area adjacent the anode, a first control grid and an accelerator grid in said first electron beam area, said control grid being provided with electron-diverting means for preventing oscillation of electrons in said first electron beam area.

This invention relates to an electron discharge device and more particularly to an electron discharge device of the gated electron beam type.

Electron discharge devices of the electron beam type such as disclosed in US. Patent No. 2,606,300'-Adler, have been found to be very useful in various receivers, particularly in television receivers, such tubes being useful as demodulator or discriminator tubes. However, such usefulness has been seriously impaired because, under certain circuit conditions, radiation was produced by these tubes causing spurious signals, thereby impairing reception by the receiver. Such spurious signals were particularly evident on certain channels; a common complaint, for example, being that Hash appears on Channel '83. It is an object of this invention to improve electron discharge device of the electron beam type so as to eliminate the radiation which causes such spurious signals.

As shall be explained more fully hereafter, it has been discovered that the radiation was generated by oscillation of the space charge in a portion of the electron beam of the tube. It is, therefore, a further object of this invention to provide an electron discharge device with means for preventing oscillation of electrons in a portion of the electron beam.

In accordance with the foregoing objects, there is provided an electron beam discharge device comprising an electron gun including an electron emitting cathode; an anode for receiving electrons emitted from said cathode; a plurality of focusing electrodes having apertures arranged in a series between said cathode and anode for shaping the electrons from said gun into an electron beam; electron accelerator means adjacent said electron beam including a limiter grid in said electron beam and delineating in said beam a first electron beam area adjacent the cathode and a second electron beam area adjacent the anode, a first control grid and an accelerator grid in said first electron beam area, the improvement which comprises electrically conductive electron diverting means in said first area for preventing oscillation of electrons in said first electron beam area.

Further objects and advantage of the invention will be understood from the following complete description and in the drawings wherein:

FIGURE 1 is a front elevation view of an electron discharge device incorporating the invention; and

FIGURE 2 is a cross-sectional view thereof.

As shown in FIGURE 1, the electron discharge device in accordance with the invention comprises a sealed envelope 3 enclosing a plurality of electrodes mounted between spaced insulating discs 5. Electrical connection to the various electrodes is made by means of base pins 7, which base pins are hermetically sealed through a base portion of the envelope 3.

As best viewed in FIGURE 2, the plurality of electrodes mounted between the insulating discs 5 comprise a source of electrons, a cathode 9 which directs electrons generally toward an anode 11. The electrons are shaped into an electron beam by a plurality of focusing electrodes including a lease electrode 13, an accelerator electrode 15, and a focus electrode 17. An accelerator grid 19 extends across the electron beam and is electrically connected to the accelerator electrode 15. A first control grid 21, usually referred to as the limiter grid, may comprise a system of grid laterals of which grid lateral 21 is one such lateral, and second control grid 23 adjacent the anode 11, which grid is usually referred to as the quadrature grid, further control the electron beam between the cathode 9 and anode 1 1 in a manner well known in the art.

For the purpose of further pointing out the invention, the electron beam may be distinguished as having two portions, a first portion R in the area between the limiter grid 21 and the cathode 9, and a second portion P between the limiter grid 21 and the anode 11. As has been pointed out hereinbefore, it has been found that the spurious signal produced by tubes of this type was caused by oscillation of the space charge of a portion of the electron beam, that portion of the electron beam causing a difficulty, being specifically the beam portion R between the cathode 9 and the limiter grid 21. To verify mathematically that the spurious signals were produced by oscillation of the space charge in this region and to determine its frequency and magitude, the following mathematical analysis was made:

Since the radiation was caused by the oscillation of charge in region occupied by the portion R, the fundamental frequency of the radiation would be the reciprocal of the transit time of an electron traveling from the cathode, through the accelerator slot, to the plane of the limiter grid, then back again to the cathode. The relation for the fundamental frequency is where u is the accelerator voltage; e is the electron charge; m is the mass of the electron; and d is the distance from the surface of the cathode to the plane of the limiter grid, which distance is approximately the axial extent of the region occupied by the portion R.

The accelerating field in region R and and the motion of an electron in that field is such that the full expression for the radiation can be expressed as the Fourier series wherein n is an interger corresponding to the fundamental (nzl) and the harmonic (n=2, 3, 4 frequencies generated.

The derived relations gave the unexpected result that the fundamental frequency was calculated to be 2.16 X 10 c.p.s., while the receiver evaluation had shown the interference to be near channels 40 and 80, which would be the 3rd and 4th harmonic frequencies. The relation indicated that the fundamental frequency should be located in the upper end of the VHF band, at channel 13, and that the intensity of the fundamental radiation should be approximately four times that of the radiation at channel 80. Further tests demonstrated that interference from the fundamental was present on channel 13 as well, although not previously noticed.

While it is, of course, possible to redesign the tube such that the distance d would result in space charge oscillation of frequency above or below the frequencies to be detected, such a solution would not be acceptable for all purposes since it would result in redesigning the tube for each particular application. It was, therefore, found that the space charge oscillation could be eliminated by forming of the limiter grid in a manner such that the field at the limiter grid would cause diversion of the electron beam in the reverse direction at cutoff. Thus, instead of the electrons returning to the area of the cathode through the opening in the accelerator electrode 15, such electrons would be collected on the accelerator electrode. Diversion of the electron beam is accomplished by shaping the limiter grid 21 with a central projecting portion 25. Thus, the difference in shape between limiter grid 21 and accelerator grid 19 is such that the field immediately adjacent the limiter grid 21 develops a sharp salient potential minimum which extends toward the cathode 9 and causes the beam to diverge when the grid 21 approaches cutoff. It will be noted from the drawing that the projecting portion 25 is generally V-shaped in hori zontal cross-section, although it is not necessary that the projecting portion be distinctly pointed, since it is the effect on the shaping of the electric field, as developed between the grid 19 and a grid 21, into a sharp point which is desired, and the mechanical distinctness of the point of the grid itself is not particularly significant.

It will, thus, be seen that there is provided an electron discharge device of the electron beam type useful for demodulator or discriminator tubes in which the oscillation of the space charge has been prevented. While the invention has been disclosed by way of the preferred embodiment thereof, it will be noted that certain modifications may be made therein without departing from the true spirit and scope of the invention as set forth in the appended claims.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

1. An electron beam discharge device comprising an electron gun including an electron emitting cathode; an anode for receiving electrons emitted from said cathode; a plurality of focusing electrodes having apertures arranged in a series between said cathode and anode for shaping the electrons from said gun into an electron beam; electron accelerator means in the path of said electron beam including a limiting grid in said electron beam and defining with said cathode a first electron beam area adjacent the cathode, and with said anode, a second electron beam area adjacent the anode, the electric field configuration in said first electron beam area being such as to tend to redirect electrons of said beam to said cathode thereby to establish unwanted oscillations, the improvement which comprises electrically conductive electron diverting means cooperating with said limiting grid for preventing oscillation of electrons in said first electron beam area.

2. An electron beam discharge device as recited in claim 1 wherein said limiting grid comprises a system of grid laterals and means for preventing oscillation comprises a central portion of the grid laterals which projects toward the cathode and effects divergence of the beam in the reverse direction.

3. An electron beam discharge device as recited in claim 2 wherein said projecting central portion of the limiting grid is generally V-shaped in cross-section whereby the electrical field developed immediately adjacent thereto is a sharp potential minimum in the center of the electron beam.

References Cited UNITED STATES PATENTS 2,983,842 5/1961 Hrbek 3l382 X 3,049,641 8/1962 Gleichauf 313--82 X 3,214,632 10/1965 Harman 31382 X 3,226,595 12/1965 Berghammer et al. 3l382 X 3,255,376 6/1966 Eichenbaum 31382 X JAMES W. LAWRENCE, Primary Examiner.

V. LAFRANCHI, Assistant Examiner.

US. Cl. X.R. 

